Solar photovoltaic devices (i.e., solar cells) are devices capable of converting solar radiation into usable electrical energy. Commonly used semiconductor solar cell devices are typically composed of one or more pairs of p-n junction cells, which include a p-type semiconductor layer adjacent an n-type semiconductor layer. Energy conversion occurs as solar radiation impinging on the solar cell and absorbed by an active region of semiconductor material generates electricity. If properly designed, multi-junction solar cells may be more efficient than single-junction solar cells, because a larger portion of the solar spectrum can be captured.
In order for the solar cell device to be economical and highly efficient, there must be an availability of high quality semiconductor materials, a flexible choice of junction band-gaps covering a broad solar spectrum, and an appropriate device architecture design that maximizes current match and minimizes electrical/optical losses. In addition, the solar cell device should minimize environmental pollution and manufacturing cost. To date, high-efficiency III-V semiconductor multi-junction solar cells have typically been grown on GaAs, InP, and Ge substrates using GaInP, and (In)GaAs cell structures to absorb solar radiation energy between 0.7 eV and 1.8 eV. Several such designs are described in U.S. Pat. Nos. 5,223,043; 5,405,453; and 5,407,491. However, significant fractions of solar radiation at wavelengths longer than 900 nm and shorter than 700 nm generally have not been effectively used due to material band gap limits in existing solar cells.
Environmental hazards are another issue with existing solar cells, such as with conventional III-V solar cell devices composed of GaAs and InGaP, which are environmentally hazardous elements after material decomposition. Also, the cost of using substrates such as GaAs and Ge is high.